ANCHORAGE - Scott Lindsey was scheduled to fly Monday in eastern interior Alaska, trying to predict the immediate future for villages along the upper Yukon River.
The hydrologist for the National Weather Service Pacific-Alaska River Forecast Center looks for signs that North America's third-longest river will overflow into communities as its frozen water breaks up and gets pushed downstream.
"In a dynamic breakup, it usually works from upstream to downstream," Lindsey said last week. "You get a big push of melt water in the Canadian Yukon. As that starts rolling down, it collects more melt water, more melt water, and creates kind of an irresistible force and just munches up the ice as it goes, and you have kind of a coherent breakup front."
Last year, thick river ice rose, broke up and jammed below three villages. The ice dammed the river and backed up river water and ice floes into Eagle, Tanana and Stevens Village.
At 37 other villages, life was disrupted by high water or precautionary evacuations, said John Madden, head of the state Division of Homeland Security and Emergency Management. The state estimated total damage at $40 million to $45 million.
"It was truly historic levels of flooding, primarily on the upper Yukon," Madden said.
The Yukon begins in British Columbia, and flows north and west through Yukon Territory before spanning more than 1,260 miles through Alaska's interior to the Bering Sea. Temperatures in the region fall below minus 40 degrees, making for thick river ice most years.
The river forecast office looks at snowfall, ice thickness and long-range temperature forecasts to assess flood potential. This year was rated as average. The snow pack in much of mainland Alaska was below average, but without a heavy blanket of snow, Yukon River ice thickness was above normal.
The main driver for flooding, however, is spring temperature. The potential increases if cool temperatures keep ice thick and are followed by sudden high temperatures, sending melt water gushing into the river channel.
Breakup started peacefully over the weekend. Ice came apart Friday at Eagle with heavy runs of river ice continuing to move Monday along the community seven miles west of the Canadian border.
By late Monday morning, the National Weather Service said an ice jam had formed 20 to 25 miles upriver from Circle, a village 125 miles northeast of Fairbanks. The agency issued a flood watch through Wednesday morning for the community.
Lindsey and a state emergency manager will watch from the air.
"What we're looking for are places where there's intact ice that looks really strong that might slow up the advance of the front," he said. "We're looking for how much water is behind the push."
A danger sign, he said, would be water a quarter mile out of bank on each side of the river above 50 miles of unbroken ice.
"Sometimes it's enough water that it can cause minor flooding just as it passes," he said. "But the bigger concern is, if it gets down around the corner from a town like Eagle and stops, it acts as a dam that constricts the portion of the river that the water can actually flow through.
Tanana and Stevens Village are perennial trouble spots.
"Stevens is just above where the river goes into the canyon, and the ice is shaded and often is harder and more difficult to push out," he said. "It's also constricted."
Islands in the river below Tanana some years constrict ice, he said.
If a community floods, the emergency manager accompanying federal hydrologists can land quickly.
"We move with the leading edge of the breakup and try to stay in advance of it," Madden said. "If we see something happening, we'll go downstream, land, and help that community."
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